The present application claims priority from copending U.S. Provisional Patent Application Ser. No. 60/358,114, filed Feb. 19, 2002.
1. Field of the Invention
The present invention relates generally to the field of gas purification. More particularly, it concerns the preparation of highly purified corrosive gases.
2. Description of Related Art
Uses for highly purified forms of a number of corrosive gases have become of interest. For example, highly purified chlorine (Cl2) is useful in fabrication of semiconductors and fiber optics components. Also, highly purified forms of hydrogen bromide (HBr) and hydrogen chloride (HCl) are useful in other applications.
Commodity grade chlorine (purity of no more than about 99.0%) is most commonly prepared by aqueous electrolysis of NaCl, yielding Cl2 vapor saturated with water vapor, NaOH, and NaCl. Other impurities present in commodity grade chlorine can include air, CO2, chlorinated hydrocarbons, COCl2, and HCl. Such impurities can be present at 1.0% or more.
In semiconductor fabrication processes using chlorine as an etching gas, the presence of as much as 5–10 ppm by volume of water vapor can degrade the performance of submicron integrated circuits. In fiber optics component fabrication processes using chlorine to remove hydrogenous impurities from molten silica, impurities present in the chlorine can degrade the optical transmission properties of a fiber formed from the molten silica.
A number of techniques for attaining higher purity levels have been reported, including distillation (Rosenblads, GB Pat. No. 1,157,238), compression liquefaction/vaporization (Payer et al., DE Pat. No. 2,926,591), organic solvent extraction (Balko et al., U.S. Pat. No. 4,230,673), adsorption using zeolites, silica gel, and chlorinated carbon (Frevel et al., U.S. Pat. No. 3,522,007; Takaishi, JP Pat. No. 52065194; Ueno et al., JP Pat. No. 58208104; Ukihashi, JP Pat. No. 55020201; and Fraenkel et al., U.S. Pat. No. 6,110,258), membrane separation (Hagg, Sep. Purif. Technol. 21:261–278 (January 2001), and electrochemical reduction/oxidation (Sarangapani et al., U.S. Pat. No. 6,203,692). Each of these techniques has shortcomings that make it difficult to quickly, efficiently, or easily prepare a highly purified gas from a process comprising the technique.
For example, distillation requires a large investment in time, equipment, and utilities to prepare a highly purified gas. Compression liquefaction/vaporization generally involves mechanical compression, which will tend to lead to high levels of metal and particulates contamination and corrosion of mechanical compression pumps by chlorine or other corrosive gases. Organic solvent extraction leads to solvent contamination, which must be removed downstream; also, many organic solvents, such as CCl4, are undesirable for use on environmental or health grounds. Adsorbents can remove specific impurities to sub-ppm levels, but not all impurities are amenable to adsorption. Membrane separation by itself is not practical for removing trace impurities. Electrochemical capture and release of chlorine is not practical for purifying high-concentration chlorine and will tend to lead to contamination by electrolysis products.
It has been shown that content of metallic impurities can be reduced by controlled vaporization of liquefied crude chlorine (Borzio et al., U.S. Pat. No. 6,004,433). However, controlled vaporization can tend to make light weight, volatile impurities more concentrated in the vapor. To remove volatile impurities from the vapor, solid adsorbers have been considered, such as so-called “acid resistant” molecular sieves, MgClO4, silica gels, and P2O5. However, these adsorbers have a number of shortcomings, e.g., MgClO4 can become explosive when in contact with organic impurities; silica gel has limited moisture removal efficiency; and P2O5 presents industrial safety issues upon reacting with water.
Therefore, there is no known purification process that provides high purity corrosive gases with a short lead time, high efficiency, and low pollution.